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Here's the thing UKbod - been very busy and stressed of late, and meaning to 'get round' haha to sorting out what seemed to be a problem - Malwarebytes (subscription) and Avast (free) seemed to be doing three rounds with each other at every boot up. In addition I had a free pdf program which was giving me concern, then - OMactualG - I tried camtasia to do some screen recording which seemed to import chromium, fiddled with my browser settings and in general made me want to drown myself. Could not find any way to delete chromium as it didn't appear on the program lists / control panel. In searching for its location in appdata folder, that's when I decided I needed to unhide folders and in doing so hid everything including firefox.
The moral of the story is never try to fix laptops when under the influence of hayfever tablets.